Dependencies: Change handling of exllamav2 checks

ExllamaV2 should check for solely exllamav2, otherwise errors don't
make sense. Migrate the combined "exl2" computed property to "inference"
since those are the required dependencies for minimal inference.

Signed-off-by: kingbri <bdashore3@proton.me>

backends/exllamav2/version.py

@@ -22,7 +22,7 @@ def check_exllama_version():
         "pip install --upgrade .[amd]\n\n"
     )
 
-    if not dependencies.exl2:
+    if not dependencies.exllamav2:
         raise SystemExit(("Exllamav2 is not installed.\n" + install_message))
 
     required_version = version.parse("0.2.2")